Method of treatment using substituted imidazo[1,2b]pyridazine compounds

ABSTRACT

Methods for treating a disease or disorder selected from pain, cancer, inflammation, neurodegenerative disease,  Typanosoma cruzi  infection and osteolytic disease in a mammal, which comprise administering to said mammal a therapeutically effective amount of a compound of Formula I 
                         
in which R 1 , R 2 , R 3 , R 4 , X, Y and n have the meanings given in the specification.

This application is a Continuation of U.S. patent application Ser. No. 13/063,894, which is a 371 National Stage filing of PCT/US2009/057729 filed Sep. 21, 2009, which claims priority to U.S. Provisional Application Ser. No. 61/099,030 filed Sep. 22, 2008, each of which is incorporated herein in its entirety.

The present invention relates to novel compounds, to pharmaceutical compositions comprising the compounds, to a process for making the compounds and to the use of the compounds in therapy. More particularly, it relates to certain substituted imidazo[1,2-b]pyridazine compounds which exhibit Trk family protein tyrosine kinase inhibition, and which are useful in the treatment of pain, inflammation, cancer and certain infectious diseases.

The current treatment regimes for pain conditions utilize several classes of compounds. The opioids (such as morphine) have several drawbacks including emetic, constipatory and negative respiratory effects, as well as the potential for addictions. Non-steroidal anti-inflammatory analgesics (NSAIDs, such as COX-1 or COX-2 types) also have drawbacks including insufficient efficacy in treating severe pain. In addition, COX-1 inhibitors can cause ulcers of the mucosa. Accordingly, there is a continuing need for new and more effective treatments for the relief of pain, especially chronic pain.

Trk's are the high affinity receptor tyrosine kinases activated by a group of soluble growth factors called neurotrophins (NT). The Trk receptor family has three members—TrkA, TrkB and TrkC. Among the neurotrophins are (i) nerve growth factor (NGF) which activates TrkA, (ii) brain-derived neurotrophic factor (BDNF) and NT-4/5 which activate TrkB and (iii) NT3 which activates TrkC. Trk's are widely expressed in neuronal tissue and are implicated in the maintenance, signaling and survival of neuronal cells (Patapoutian, A. et al., Current Opinion in Neurobiology, 2001, 11, 272-280).

Inhibitors of the Trk/neurotrophin pathway have been demonstrated to be effective in numerous pre-clinical animal models of pain. For example, antagonistic TrkA/NGF pathway antibodies (for example, RN-624) have been shown to be efficacious in inflammatory and neuropathic pain animal models and in human clinical trials (Woolf, C. J. et al. (1994) Neuroscience 62, 327-331; Zahn, P. K. et al. (2004) J. Pain 5, 157-163; McMahon, S. B. et al., (1995) Nat. Med. 1, 774-780; Ma, Q. P. and Woolf, C. J. (1997) Neuroreport 8, 807-810; Shelton, D. L. et al. (2005) Pain 116, 8-16; Dclafoy, L. et al. (2003) Pain 105, 489-497; Lamb, K. et al. (2003) Neurogastroenterol. Motil. 15, 355-361; Jaggar, S. I. et al. (1999) Br. J. Anaesth. 83, 442-448). Additionally, recent literature indicates after inflammation, BDNF levels and TrkB signaling is increased in the dorsal root ganglion (Cho, L. et al. Brain Research 1997, 749, 358) and several studies have show antibodies that decrease signaling through the BDNF/TrkB pathway inhibit neuronal hypersensitization and the associated pain (Chang-Qi, L et al. Molecular Pain 2008, 4:27)

In addition it was shown that tumor cells and tumor invading macrophages secret NGF which directly stimulates TrkA located on peripheral pain fibers. Using various tumor models in both mouse and rats it was demonstrated that neutralizing NGF with a monoclonal antibody inhibits cancer related pain to a degree similar or superior to the highest tolerated dose of morphine. Therefore, an inhibitor of TrkA can be used in the treatment of pain, including pain associated with cancer.

Recent literature has also shown that overexpression, activation, amplification and/or mutation of Trks are associated with many cancers including neuroblastoma (Brodeur, G. M., Nat. Rev. Cancer 2003, 3, 203-216), ovarian cancer (Davidson. B., et al., Clin. Cancer Res. 2003, 9, 2248-2259), breast cancer (Kruettgen et al, Brain Pathology 2006, 16: 304-310), prostate cancer (Dionne et al, Clin. Cancer Res. 1998, 4(8): 1887-1898), pancreatic cancer (Dang et al, Journal of Gastroenterology and Hepatology 2006, 21(5): 850-858), multiple myeloma (Hu et al, Cancer Genetics and Cytogenetics 2007, 178: 1-10), astrocytoma amd medulloblastoma (Kruettgen et al, Brain Pathology 2006, 16: 304-310) glioma (Hansen et al, Journal of Neurochemistry 2007, 103: 259-275), melanoma (Truzzi et al, Journal of Investigative Dermatology 2008, 128(8): 2031-2040, thyroid carcinoma (Brzezianska et al, Neuroendocrinology Letters 2007, 28(3), 221-229.), lung adenocarcinoma (Perez-Pinera et al, Molecular and Cellular Biochemistry 2007, 295(1&2), 19-26), large cell neuroendocrine tumors (Marchetti et al, Human Mutation 2008, 29(5), 609-616), and colorectal cancer (Bardelli, A., Science 2003, 300, 949). In preclinical models of cancer, non-selective small molecule inhibitors of Trk A, B and C and Trk/Fc chimeras were efficacious in both inhibiting tumor growth and stopping tumor metastasis (Nakagawara, A. (2001) Cancer Letters 169:107-114; Meyer, J. et al. (2007) Leukemia, 1-10; Pierottia, M. A. and Greco A., (2006) Cancer Letters 232:90-98; Eric Adriaenssens, E. et al. Cancer Res (2008) 68:(2) 346-351) (Truzzi et al, Journal of Investigative Dermatology 2008, 128(8): 2031-2040.

In addition, inhibition of the neurotrophin/Trk pathway has been shown to be effective in treatment of pre-clinical models of inflammatory disease. For example inhibition of the neurotrophin/Trk pathway has been implicated preclinical models of inflammatory lung disease including asthma (Freund-Michel, V; Frossard, N.; Pharmacology & Therapeutics (2008), 117(1), 52-76), interstitial cystitis (Hu Vivian Y; et. al. The Journal of Urology (2005), 173(3), 1016-21), inflammatory bowel disease including ulcerative colitis and Crohn's disease (Di Mola, F. F, et. al., Gut (2000), 46(5), 670-678) and inflammatory skin diseases such as atopic dermatitis (Dou, Y.-C.; et. al. Archives of Dermatological Research (2006), 298(1), 31-37), eczema and psoriasis (Raychaudhuri, S. P.; et. al. Journal of Investigative Dermatology (2004), 122(3), 812-819).

The neurotrophin/Trk pathway, particularly BDNF/TrkB, has also been implicated in the etiology neurodegenerative diseases including multiple sclerosis, Parkinson's disease and Alzheimer's disease (Sohrabji, Farida; Lewis, Danielle K. Frontiers in Neuroendocrinology (2006), 27(4), 404-414). Modulation of the neutrophin/Trk pathway may have utility in treatment of these and related diseases.

The TrkA receptor is also thought to be critical to the disease process in the infection of the parasitic infection of Typanosoma cruzi (Chagas disease) in human hosts (de Melo-Jorge, M. et al. Cell Host & Microbe (2007), 1(4), 251-261). Thus, TrkA inhibition my have utility in treating Chagas disease and related protozoan infections.

Trk inhibitors may also find use in treating disease related to an imbalance of the regulation of bone remodeling, such as osteoporosis, rheumatoid arthritis and bone metastases. Bone metastases are a frequent complication of cancer, occurring in up to 70 percent of patients with advanced breast or prostate cancer (1) and in approximately 15 to 30 percent of patients with carcinoma of the lung, colon, stomach, bladder, uterus, rectum, thyroid, or kidney. Osteolytic metastases can cause severe pain, pathologic fractures, life-threatening hypercalcemia, spinal cord compression, and other nerve-compression syndromes. For these reasons, bone metastasis is a serious and costly complication of cancer. Therefore, agents that can induce apoptosis of proliferating osteoblasts would be highly advantageous. Expression of TrkA and TrkC receptors has been observed in the bone forming area in mouse models of bone fracture (K. Asaumi, et al., Bone (2000) 26(6) 625-633). In addition, localization of NGF was observed in almost all bone forming cells (K. Asaumi, et al.). Recently, it was demonstrated that a pan-Trk inhibitor inhibits the tyrosine signaling activated by neurotrophins binding to all three of the Trk receptors in human hFOB osteoblasts (J. Pinski, et al., (2002) 62, 986-989). These data support the rationale for the use of Trk inhibitors for the treatment of bone remodeling diseases, such as bone metastases in cancer patients.

Several classes of small molecule inhibitors of Trk kinases said to be useful for treating pain or cancer are known (Expert Opin. Ther. Patents (2009) 19(3)). International Patent Application Publications WO 2006/115452 and WO 2006/087538 describe several classes of small molecules said to be inhibitors or Trk kinases which could be useful for treating pain or cancer.

U.S. Patent Publication number 2007/025540 discloses certain substituted imidazo[1,2b]pyridazines having a secondary amino group or a BOC-protected piperazinyl group at the 6-position. These compounds are disclosed as being inhibitors of the protein kinase C (PKC).

International Publication No. WO 2008/052734 discloses (R)-4-(6-(2-(3-fluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)benzonitrile, that is, an imidazo[1,2b]pyridazine compound bearing an aryl-substituted heterocyclic group at the 6-position and a benzonitrile group at the 3 position. This compound does not fall within the general formulae disclosed therein representing 3-aryl substitutedimidazo[1,2-b]pyridazines. This compound is purported to be suitable for treating diseases mediated by the PI3K receptor, the JAK-2 receptor and the Trk receptor.

International Publication No. WO 2007/013673 discloses 1-phenyl-3-(6-(1-phenylethylamino)imidazo[1,2-b]pyridazin-3-yl)urea and N-(6-(4-hydroxycyclohexylamino) imidazo[1,2-b]pyridazin-3-yl)benzamide, that is, imidazo[1,2b]pyridazine compounds bearing an amino group at the 6-position and an amide or urea moiety at the 3 position. These compounds are said to be Lck inhibitors.

There is a continuing need for new and more effective treatments for the relief of pain, especially chronic pain. Because TrkA and other Trk kinases may serve as a mediator of NGF driven biological responses, inhibitors of TrkA and other Trk kinases may provide an effective treatment for chronic pain states.

It has now been found that certain imidazo[1,2b]pyridazine compounds bearing an aryl or heteroaryl-substituted heterocyclic group at the 6-position and a group having the formula NR′C(═O)R² at the 3-position, wherein R¹ and R² are as defined herein, are inhibitors of Trk kinases, in particular inhibitors of TrkA and/or TrkB, which are useful for treating disorders and diseases which can be treated by inhibiting Trk-A and/or TrkB kinases, such as pain, including chronic and acute pain, or cancer. Certain compounds of the invention which are inhibitors of TrkA and/or TrkB may be useful in the treatment of multiple types of pain (including acute and chronic pain) including inflammatory pain, neuropathic pain, and pain associated with cancer, surgery and bone fracture. In addition, compounds of the invention may be useful for treating cancer, inflammation, neurodegenerative diseases and certain infectious diseases.

Accordingly, one embodiment of this invention provides a compound of the general Formula I:

or a pharmaceutically acceptable salt thereof, wherein:

R¹ is H or (1-6C alkyl);

R² is NR^(b)R^(c), (1-4C)alkyl, (1-4C)fluoro alkyl, CF₃, (1-4C)hydroxyalkyl, -(1-4C alkyl)hetAr¹, -(1-4C alkyl)NH(1-4C alkyl), hetAr², hetCyc¹, hetCyc², phenyl which is optionally substituted with NHSO₂(1-4C alkyl), or (3-6C)cycloalkyl which is optionally substituted with (1-4C alkyl), CN, OH, CF₃, CO₂(1-4C alkyl) or CO₂H;

R^(b) is H or (1-6C alkyl);

R^(c) is H, (1-4C)alkyl, (1-4C)hydroxyalkyl, hetAr³, or phenyl, wherein said phenyl is optionally substituted with one or more substituents independently selected from halogen, CN, CF₃ and —O(1-4C alkyl),

or NR^(b)R^(c) forms a 4 membered heterocyclic ring having a ring nitrogen atom wherein said heterocyclic ring is optionally substituted with one or more substituents independently selected from halogen, OH, (1-4C alkyl), (1-4C)alkoxy, —OC(═O)(1-4C alkyl), NH₂, —NHC(═O)O(1-4C alkyl), and (1-4C)hydroxyalkyl,

or NR^(b)R^(c) forms a 5-6 membered heterocyclic ring having a ring heteroatom which is nitrogen and optionally having a second ring heteroatom or group selected from N, O and SO₂, wherein the heterocyclic ring is optionally substituted with one or more substituents independently selected from OH, halogen, CF₃, (1-4C)alkyl, CO₂(1-4C alkyl), CO₂H, NH₂, NHC(═O)O(1-4C alkyl) and oxo,

or NR^(b)R^(c) forms a 7-8 membered bridged heterocyclic ring having 1-2 ring nitrogen atoms and optionally substituted with CO₂(1-4C alkyl);

hetAr¹ is a 5-membered heteroaryl ring having 1-3 ring nitrogen atoms;

hetAr² is 5-6 membered heteroaryl ring having at least one nitrogen ring atom and optionally having a second ring heteroatom independently selected from N and S, wherein said heteroaryl ring is optionally substituted with one or more substituents independently selected from (1-4C alkyl), halogen, -(1-4C)alkoxy, and NH(1-4C alkyl);

hetCyc¹ is a carbon-linked 4-6 membered azacyclic ring optionally substituted with one or more substituents independently selected from (1-4C alkyl), CO₂H and CO₂(1-4C alkyl);

hetCyc² is a pyridinone or pyridazinone ring substituted with a substituent selected from (1-4C)alkyl;

hetAr³ is a 5-6 membered heteroaryl ring having 1-2 ring heteroatoms independently selected from N and O and optionally substituted with one or more substituents independently selected from (1-4C)alkyl;

Y is a phenyl ring optionally substituted with one or more substituents independently selected from halogen, (1-4C)alkoxy, CF₃ and CHF₂, or a 5-6 membered heteroaryl ring having a ring heteroatom selected from N and S;

X is null, —CH₂—, —CH₂CH₂—, —CH₂O—, or —CH₂NR^(d)—;

R^(d) is H or (1-4C alkyl);

R³ is H or (1-4C alkyl);

each R⁴ is independently selected from halogen, (1-4C)alkyl, OH, (1-4C)alkoxy, NH₂, NH(1-4C alkyl) and CH₂OH; and

n is 0, 1, 2, 3, 4, 5 or 6.

In certain embodiments of Formula I, R¹ is hydrogen.

In certain embodiments of Formula I, R¹ is (1-6C)alkyl. A particular example is methyl.

In certain embodiments of Formula I, R² is a group having the formula NR^(b)R^(c), such that the group at the 3 position of the imidazo[1,2b]pyridazine core of Formula I has the formula —NR¹C(═O)NR^(b)R^(c).

In certain embodiments, R^(b) is H. In certain embodiments, R^(b) is (1-6C alkyl), for example Me. In certain embodiments, R^(c) is H, (1-4C)alkyl, (1-4C)hydroxyalkyl, hetAr³, or phenyl, wherein said phenyl is optionally substituted with one or more substituents independently selected from halogen, CN, CF₃ and —O(1-4C alkyl).

In certain embodiments, R² is NR^(b)R^(c), where R^(c) is hydrogen. In particular embodiments, the group represented by NR^(b)R^(c) is NH₂.

In certain embodiments, R² is NR^(b)R^(c), where R^(c) is (1-4C)alkyl. Examples include methyl, ethyl, propyl, isopropyl, butyl, isobutyl, and the like. In particular embodiments, the group represented by NR^(b)R^(c) includes NHMe, NMe₂ and NH(t-butyl).

In certain embodiments, R² is NR^(b)R^(c), where R^(c) is (1-4C)hydroxyalkyl. Examples include CH₂CH₂OH and CH₂CH₂CH₂OH. In particular embodiments, the group represented by NR^(b)R^(c) includes NMe(CH₂CH₂OH).

In certain embodiments, R² is NR^(b)R^(c), where R^(c) is hetAr³, and hetAr³ is an optionally substituted 5-6 membered heteroaryl ring having 1-2 ring heteroatoms independently selected from N and O. An example of hetAr³ includes an isoxazolyl ring. In certain embodiments, hetAr³ is unsubstituted. In other embodiments, hetAr³ is substituted with one or more substituents independently selected from (1-4C)alkyl, for example one or more substituents independently selected from methyl and ethyl. Examples of hetAr³ include dimethylisoxazolyl. In particular embodiments, the group represented by NR^(b)R^(c) includes the group having the structure:

In certain embodiments, R² is NR^(b)R^(c), where R^(c) is a phenyl group optionally substituted with one or more substituents independently selected from halogen, CN, CF₃ and O-(1-4C alkyl). Examples of R^(c) include phenyl, fluorophenyl, chlorophenyl, cyanophenyl, methoxyphenyl, trifluoromethylphenyl, dichlorophenyl, and trimethoxyphenyl. More particular examples include 4-fluorophenyl, 3-chlorophenyl, 4-chlorophenyl, 3-cyanophenyl, 4-cyanophenyl, 4-methoxyphenyl, 2-4-dichlorophenyl, 3-(trifluoromethyl)phenyl, 3,5-di chlorophenyl, and 3,4,5-trimethoxyphenyl. In particular embodiments, the group represented by NR^(b)R^(c) includes groups having the structures:

In certain embodiments, R² is —NR^(b)R^(c), wherein:

(i) NR^(b)R^(c) forms a 4 membered heterocyclic ring having a ring nitrogen atom optionally substituted with one or more substituents independently selected from halogen, OH, (1-4C alkyl), (1-4C)alkoxy, —OC(═O)(1-4C alkyl), NH₂, —NHC(═O)O(1-4C alkyl), and (1-4C)hydroxyalkyl, or

(ii) NR^(b)R^(c) forms a 5-6 membered heterocyclic ring having a ring heteroatom which is nitrogen and optionally having a second ring heteroatom or group selected from N, O and SO₂, wherein the heterocyclic ring is optionally substituted with one or more substituents independently selected from OH, halogen, CF₃, (1-4C)alkyl, CO₂(1-4C alkyl), CO₂H, NH₂, NHC(═O)O(1-4C alkyl) and oxo, or

(iii) NR^(b)R^(c) forms a 7-8 membered bridged heterocyclic ring having 1-2 ring nitrogen atoms and optionally substituted with CO₂(1-4C alkyl).

In certain embodiments, R² is —NR^(b)R^(c), wherein —NR^(b)R^(c) forms a 4 membered heterocyclic ring having a ring nitrogen atom and which is optionally substituted with one or more substituents independently selected from F, OH, (1-4C alkyl), —O(1-4C alkyl), —OC(═O)(1-4C alkyl), NH₂, —NHC(═O)O(1-4C alkyl), and (1-4C)hydroxyalkyl. Examples include azetidinyl rings optionally substituted with one or more groups independently selected from OH, methyl, OMe, OC(═O)C(CH₃)₂, NH₂, —NHC(═O)OC(CH₃)₃ and CH₂OH. Particular examples of R² when represented by —NR^(b)R^(c), wherein —NR^(b)R^(c) forms a 4 membered heterocyclic ring, include the structures:

In certain embodiments, R² is —NR^(b)R^(c), wherein —NR^(b)R^(c) forms a 5-6 membered heterocyclic ring having a ring heteroatom which is nitrogen and optionally having a second ring heteroatom or group selected from N, O and SO₂, wherein the heterocyclic ring is optionally substituted with one or more substituents independently selected from OH, halogen, CF₃, (1-4C)alkyl, CO₂(1-4C alkyl), CO₂H, NH₂, NHC(═O)O(1-4C alkyl) and oxo. Examples include optionally substituted pyrrolidinyl, piperidinyl, piperazinyl, morpholinyl and piperidinesulfone rings. Examples of substituents on the 5-6 membered heterocyclic ring include OH, F, NH₂, CO₂H, CO₂Et, NHCO₂C(CH₃)₃, CF₃, methyl, ethyl, isopropyl, CO₂C(CH₂)₃ and oxo. In one embodiment, the heterocyclic ring is optionally substituted with one or two of said substituents. Particular examples of R² when represented by —NR^(b)R^(c), wherein —NR^(b)R^(c) forms a 5-6 membered heterocyclic ring, include the structures:

In certain embodiments, R² is —NR^(b)R^(c), wherein NR^(b)R^(c) forms a 7-8 membered bridged heterocyclic ring having 1-2 ring nitrogen atoms and optionally substituted with CO₂(1-4C alkyl). Examples of bridged heterocyclic rings include diazabicyclooctane rings such as 3,8-diazabicyclo[3.2.1]octane rings, which are optionally substituted with CO₂(1-4C alkyl), such as CO₂C(CH₃)₃. Particular examples of R² when represented by —NR^(b)R^(c), wherein —NR^(b)R^(c) forms a 7-8 membered bridged heterocyclic ring, include the structures:

In certain embodiments, R² is selected from (1-4C)alkyl, (1-4C)fluoroalkyl, CF₃, -(1-4C alkyl)hetAr¹, and -(1-4C alkyl)NH(1-4C alkyl). In certain embodiments, R² is selected from (1-4C)alkyl, (1-4C)fluoroalkyl, CF₃, -(1-4C)hydroxyalkyl, (1-4C alkyl)hetAr¹, and -(1-4C alkyl)NH(1-4C alkyl).

In certain embodiments, R² is (1-4C)alkyl. Particular examples include methyl, isopropyl and tert-butyl.

In certain embodiments, R² is (1-4C)fluoroalkyl. A particular example includes CF(CH₃)₂.

In certain embodiments, R² is CF₃.

In certain embodiments, R² is (1-4C)hydroxyalkyl. Particular examples include C(CH₃)₂OH and C(CH₃)₂CH₂OH.

In certain embodiments, R² is (3-6C cycloalkyl) which is optionally substituted with (1-4C)alkyl, CN, OH, CF₃, CO₂(1-4C alkyl) or CO₂H. In certain embodiments, R² is an optionally substituted cyclopropyl ring. Particular examples of R² include the structures:

In certain embodiments, R² is -(1-4C alkyl)hetAr¹, where hetAr¹ is a 5-membered heteroaryl ring having 1-3 ring nitrogen atoms. An example of hetAr¹ is a triazolyl ring, such as 1,2,4-triazolyl. Examples of the (1-4C)alkyl portion include methylene, ethylene, dimethylmethylene, and the like. A particular value for R² when represented by -(1-4C alkyl)hetAr¹ is the structure:

In certain embodiments, R² is -(1-4C alkyl)NH(1-4C alkyl). Examples include groups having the formula (1-4C alkyl)NHCH₃. A particular value include —C(CH₃)₂NHCH₃.

In certain embodiments, R² is selected from hetAr², hetCyc¹, hetCyc² and hetAr³. In certain embodiments, R² is selected from hetAr², hetCyc¹, and hetCyc².

In certain embodiments, R² is hetAr². Examples of hetAr² include pyridyl, pyrimidyl, pyrazinyl, pyrazolyl, imidazolyl and thiazolyl rings optionally substituted with one or more substituents independently selected from (1-4C alkyl), halogen, —O(1-4C alkyl), and NH(1-4C alkyl). Examples of substituents for hetAr² include methyl, ethyl, chloro, OMe, and NHCH(CH₃)₂. Particular values of R² include the structures:

In certain embodiments, R² is hetCyc¹. Examples of hetCyc¹ include carbon-linked azetidinyl, pyrrolidinyl and piperidinyl rings optionally substituted with one or more substituents independently selected from (1-4C alkyl), CO₂H and CO₂(1-4C alkyl). Examples of substituents include methyl, ethyl, propyl, CO₂H, CO₂Me, CO₂Et, and CO₂C(CH₃)₃. In one embodiment, hetCyc¹ is optionally substituted with one or two of said substituents. Particular values for R² represented by hetCyc¹ include the structures:

In certain embodiments, R² is hetCyc². Examples include pyridinone or pyridazinone ring substituted with a substituent selected from (1-4C)alkyl such as a methyl or ethyl group. Particular values of R² when represented by hetCyc² include the structures:

In certain embodiments, R² is phenyl which is optionally substituted with an NHSO₂(1-4C alkyl) group such a methanesulfonamido or an ethanesulfonamido group. Particular values for R² include the structures:

Referring now to the substituents on the ring at the 6-position of Formula I, in one embodiment Y is phenyl optionally substituted with one or more substituents independently selected from halogen, (1-4C)alkoxy, CF₃ and CHF₂. In one embodiment, Y is phenyl optionally substituted with one or more substituents independently selected from F, Cl, OMe, CF₃ and CHF₂. In certain embodiments, Y is phenyl optionally substituted with one or two of said substituents. Particular values for Y include phenyl, 3-fluorophenyl, 2,5-difluorophenyl, 2-chloro-5-fluorophenyl, 2-methoxyphenyl, 2-methoxy-5-fluorophenyl, 2-trifluoromethyl-5-fluorophenyl, 2-difluoromethyl-5-fluorophenyl and 3-chloro-5-fluorophenyl.

In one embodiment, Y is a 5-6 membered heteroaryl ring having a ring heteroatom selected from N and S. Examples include pyridyl and thienyl groups. Particular values for Y include 2-pyridyl, 3-pyridyl and 2-thienyl.

In one embodiment, the Y group has the absolute configuration shown in FIG. Ia:

With reference to the R³ substituent, in one embodiment R³ is H. In one embodiment, R³ is (1-4C)alkyl, for example, methyl, ethyl, propyl, isopropyl, or butyl. Particular values for R³ include hydrogen and methyl.

With reference to the R⁴ substituent, in one embodiment R⁴ is halogen. Particular examples are fluoro and chloro.

In one embodiment, R⁴ is (1-4C)alkyl, such as methyl, ethyl, propyl, isopropyl, or butyl. A particular example is methyl.

In one embodiment, R⁴ is OH.

In one embodiment, R⁴ is (1-4C)alkoxy, for example OMe and OEt.

In one embodiment, R⁴ is NH₂.

In one embodiment, R⁴ is NH(1-4C alkyl), for example NHMe, NHEt, NHPr, NHiPr and NHBu. A particular example is NHMe.

In one embodiment, R⁴ is CH₂OH.

In one embodiment, each R⁴ is independently selected from F, Cl, OH, OMe, NH₂, Mc, CH₂OH, and NHMe.

In one embodiment, n is 0, 1, 2, 3 or 4. In one embodiment, n is 0, 1, 2 or 3. In one embodiment, n is 0, 1 or 2.

With continued reference to the ring at the 6-position of Formula I, in certain embodiments, X is null, —CH₂— or —CH₂CH₂—.

In one embodiment X is null, such that the heterocyclic ring at the 6-position of Formula I has the structure:

where R³, R⁴, Y and n are as defined herein. In one embodiment, Y is phenyl optionally substituted with one or two F. In one embodiment, Y is a 5-6 membered heteroaryl ring. In one embodiment, R³ is hydrogen. In another embodiment, R³ is methyl. A particular example of the ring at the 6-position of Formula I when X is null includes the structures:

In one embodiment, X is CH₂, such that the heterocyclic ring at the 6-position of Formula I has the structure:

where R³, R⁴, Y and n are as defined herein. In one embodiment Y is phenyl substituted with one or two fluoro atoms. In one embodiment, R³ is hydrogen. In another embodiment, R³ is methyl. In one embodiment, each R⁴ is independently selected from F, Cl, Me, OH, OMe, NH₂, NHMe, CH₂OH, CHF₂ and CF₃. In one embodiment, n is 0, 1 or 2. Particular examples of the ring at the 6-position of Formula I when X is CH₂ include the structures:

In one embodiment, X is CH₂, such that the heterocyclic ring at the 6-position of Formula I has the structure:

where R³, R⁴, Y and n are as defined herein. In one embodiment, Y is a 5-6 membered heteroaryl ring having a ring heteroatom selected from N and S. Examples of 5-6 membered heteroaryl rings include pyridyl and thienyl. In one embodiment, R³ is hydrogen. In another embodiment, R³ is methyl. In one embodiment, each R⁴ is independently selected from F, Cl, Me, OH, OMe, NH₂, NHMe and CH₂OH. In one embodiment, n is 0, 1 or 2. In one embodiment, n is 0. Particular examples of the ring at the 6-position of Formula I when X is CH₂ include the structures:

In one embodiment, X is CH₂CH₂, such that the heterocyclic ring at the 6-position of Formula I has the structure:

where R³, R⁴, Y and n are as defined herein. In one embodiment, Y is phenyl optionally substituted with one or two fluoro atoms. In one embodiment, Y is a pyridyl ring. In one embodiment, R³ is hydrogen. In another embodiment, R³ is methyl. In one embodiment, n is 0, 1 or 2. In one embodiment, n is 0. Particular examples of the ring at the 6-position of Formula I when X is CH₂CH₂ include the structures:

In one embodiment, X is —CH₂O—, such that the heterocyclic ring at the 6-position of Formula I has the structure:

where R³, R⁴, Y and n are as defined herein. In one embodiment, Y is phenyl optionally substituted with one or more substituents independently selected from F and (1-4C)alkoxy, for example one or two substituents independently selected from F and OMe. In one embodiment, Y is fluorophenyl, difluorophenyl or methoxyphenyl. In one embodiment, Y is pyridyl. In one embodiment, R³ is hydrogen. In another embodiment, R³ is methyl. In one embodiment, n is 0, 1 or 2. Particular examples of the ring at the 6-position of Formula I when X is —CH₂O— include the structures:

In one embodiment, X is —CH₂NR^(d)—, such that the heterocyclic ring at the 6-position of Formula I has the structure:

where R³, R⁴, Y, R^(d) and n are as defined herein. In one embodiment, R^(d) is H. In one embodiment, R^(d) is (1-4C alkyl), for example methyl, ethyl, propyl, isopropyl, or butyl. A particular example is methyl. In one embodiment, Y is phenyl optionally substituted with one or two F. In one embodiment, n is 0. Particular examples of the ring at the 6-position of Formula I when X is —CH₂NR^(d)— include the structures:

Compounds of Formula I include compound of Formula Ib, wherein:

R¹ is H or (1-6C alkyl);

R² is NR^(b)R^(c);

NR^(b)R^(c) forms a 4 membered heterocyclic ring having a ring nitrogen atom, wherein said heterocyclic ring is optionally substituted with one or more substituents independently selected from halogen, OH, (1-4C alkyl), (1-4C)alkoxy, —OC(═O)(1-4C alkyl), NH₂, —NHC(═O)O (1-4C alkyl) and (1-4C)hydroxyalkyl,

or NR^(b)R^(c) forms a 5-6 membered heterocyclic ring having a ring heteroatom which is nitrogen and optionally having a second ring heteroatom or group selected from N, O and SO₂, wherein the heterocyclic ring is optionally substituted with one or more substituents independently selected from OH, halogen, CF₃, (1-4C)alkyl, CO₂(1-4C alkyl), CO₂H, NH₂, NHC(═O)O(1-4C alkyl) and oxo;

Y is phenyl optionally substituted with one or more substituents independently selected from halogen, (1-4C)alkoxy, CF₃ and CHF₂;

X is null, —CH₂—, or —CH₂CH₂—;

R³ is H or (1-4C alkyl);

each R⁴ is independently selected from halogen, (1-4C)alkyl, OH, (1-4C)alkoxy, NH₂, NH(1-4C alkyl) and CH₂OH; and

-   -   n is 0, 1, or 2.

In one embodiment of Formula Ib, Y is phenyl optionally substituted with one or more halogen atoms. In one embodiment of Formula Ib, Y is phenyl optionally substituted with one or two fluorine atoms.

In one embodiment of Formula Ib, (i) NR^(b)R^(c) forms a 4 membered heterocyclic ring having a ring nitrogen atom, wherein said ring is optionally substituted with one or more substituents independently selected from halogen, OH, (1-4C alkyl), (1-4C)alkoxy, —OC(═O)(1-4C alkyl), NH₂, —NHC(═O)O(1-4C alkyl) and (1-4C)hydroxyalkyl, or (ii) NR^(b)R^(c) forms a 5-6 membered heterocyclic ring having a ring heteroatom which is nitrogen and optionally having a second ring heteroatom or group selected from N, O and SO₂, wherein the heterocyclic ring is optionally substituted with one or more substituents independently selected from OH, halogen, CF₃, (1-4C)alkyl, CO₂(1-4C alkyl), CO₂H, NH₂, NHC(═O)O(1-4C alkyl) and oxo.

In one embodiment of Formula Ib, n is zero or one.

In one embodiment of Formula Ib, R³ is hydrogen.

Compounds of Formula Ib include compounds of Formula Ic wherein:

R¹ is H or (1-6C alkyl);

R² is NR^(b)R^(c);

NR^(b)R^(c) forms a 4 membered heterocyclic ring having a ring nitrogen atom, wherein said heterocyclic ring is optionally substituted with one or more substituents independently selected from halogen, OH, (1-4C alkyl), (1-4C)alkoxy, —OC(═O)(1-4C alkyl), NH₂, —NHC(═O)O(1-4C alkyl) and (1-4C)hydroxyalkyl;

Y is phenyl optionally substituted with one or more substituents independently selected from halogen, (1-4C)alkoxy, CF₃ and CHF₂;

X is —CH₂—;

R³ is H or (1-4C alkyl);

each R⁴ is independently selected from halogen, (1-4C)alkyl, OH, (1-4C)alkoxy, NH₂, NH(1-4C alkyl) and CH₂OH; and

n is 0, 1, or 2.

In one embodiment of Formula Ic, the heterocyclic ring formed by NR^(b)R^(c) is optionally substituted with one or two substituents independently selected from F, OH, methyl, OMc, OC(═O)C(CH₃)₂, NH₂, —NHC(═O)OC(CH₃)₃ and CH₂OH.

In one embodiment of Formula Ic, Y is phenyl optionally substituted with one or more halogen atoms. In one embodiment of Formula Ic, Y is phenyl optionally substituted with one or two fluorine atoms.

Compounds of Formula Ib also include compounds of Formula Id wherein:

R¹ is H or (1-6C alkyl);

R² is NR^(b)R^(c);

NR^(b)R^(c) forms a 5-6 membered heterocyclic ring having a ring heteroatom which is nitrogen and optionally having a second ring heteroatom or group selected from N, O and SO₂, wherein the heterocyclic ring is optionally substituted with one or more substituents independently selected from OH, halogen, CF₃, (1-4C)alkyl, CO₂(1-4C alkyl), CO₂H, NH₂, NHC(═O)O(1-4C alkyl) and oxo;

Y is phenyl optionally substituted with one or more substituents independently selected from halogen, (1-4C)alkoxy, CF₃ and CHF₂;

X is —CH₂—;

R³ is H or (1-4C alkyl);

each R⁴ is independently selected from halogen, (1-4C)alkyl, OH, (1-4C)alkoxy, NH₂, NH(1-4C alkyl) and CH₂OH; and

n is 0, 1, or 2.

In one embodiment of Formula Id, the heterocyclic ring formed by NR^(b)R^(c) is optionally substituted with one or two substituents independently selected from OH, F, NH₂, CO₂H, CO₂Et, NHCO₂C(CH₃)₃, CF₃, methyl, ethyl, isopropyl, CO₂C(CH₃)₃ and oxo.

In one embodiment of Formula Id, the heterocyclic ring formed by NR^(b)R^(c) is a 5-6 membered azacyclic ring.

In one embodiment of Formula Id, Y is phenyl optionally substituted with one or more halogen atoms. In one embodiment of Formula Id, Y is phenyl optionally substituted with one or two fluorine atoms.

In one embodiment of Formula Ic or Id, n is zero or one.

In one embodiment of Formula Ic or Id, R³ is hydrogen.

In one embodiment of Formula Ic or Id, R¹ is hydrogen.

It will be appreciated that certain compounds according to the invention may contain one or more centers of asymmetry and may therefore be prepared and isolated in a mixture of isomers such as a racemic mixture, or in an enantiomerically pure form. It is intended that all stereoisomeric forms of the compounds of the invention, including but not limited to, diastereomers, enantiomers and atropisomers, as well as mixtures thereof such as racemic mixtures, form part of the present invention.

In the structures shown herein, where the stereochemistry of any particular chiral atom is not specified, then all stereoisomers are contemplated and included as the compounds of the invention. Where stereochemistry is specified by a solid wedge or dashed line representing a particular configuration, then that stereoisomer is so specified and defined.

It will also be appreciated that certain compounds of Formula I may be used as intermediates for further compounds of Formula I.

The compounds of Formula I include pharmaceutically acceptable salts thereof. In addition, the compounds of Formula I also include other salts of such compounds which are not necessarily pharmaceutically acceptable salts, and which may be useful as intermediates for preparing and/or purifying compounds of Formula I and/or for separating enantiomers of compounds of Formula I. Particular examples include hydrochloride and trifluoroacetate salts of compounds of Formula I.

It will further be appreciated that the compounds of Formula I or their salts may be isolated in the form of solvates, and accordingly that any such solvate is included within the scope of the present invention.

The term “(1-4C) alkyl” as used herein refers to saturated linear or branched-chain monovalent hydrocarbon radicals of one to four carbon atoms, respectively. Examples include, but are not limited to, methyl, ethyl, 1-propyl, 2-propyl, 1-butyl, 2-methyl-1-propyl, 2-butyl, and 2-methyl-2-propyl.

The term “(1-4C) alkoxy” as used herein refers to saturated linear or branched-chain monovalent radicals of one to four carbon atoms, respectively, wherein the radical is on the oxygen atom.

The term “(1-4C)hydroxyalkyl” as used herein refers to saturated linear or branched-chain monovalent hydrocarbon radicals of one to four carbon atoms, respectively, wherein one of the hydrogen atoms is replaced with an OH group.

The term “halogen” includes fluoro, chloro, bromo and iodo.

According to another aspect, the present invention provides a process for the preparation of a compound of Formula I or a pharmaceutically acceptable salt thereof as defined herein which comprises:

-   -   (a) for a compound of Formula I wherein R² is NR^(b)R^(c),         reacting a corresponding compound of formula II

-   -   with a compound having the formula HNR^(b)R^(c) in the presence         of a coupling reagent; or     -   (b) for a compound of Formula I wherein R² is NR^(b)R^(c) and         R^(b) is H, reacting a corresponding compound of formula II with         a compound having the formula O═C═N—R^(c); or     -   (c) for a compound of Formula I wherein R² is hetAr² or a phenyl         ring which is optionally substituted with NHSO₂(1-4C alkyl),         reacting a corresponding compound of Formula II with a         corresponding compound having the formula HOC(═O)R² in the         presence of a coupling reagent and a base; or     -   (d) for a compound of Formula I wherein R² is (1-4C)alkyl,         (1-4C)fluoroalkyl, CF₃, (1-4C)hydroxyalkyl, or (3-6C)cycloalkyl         which is optionally substituted with (1-4C alkyl), CN, OH, CF₃,         CO₂(1-4C alkyl) or CO₂H, reacting a corresponding compound of         Formula II with a corresponding compound having the formula         (R²CO)₂O in the presence of a base; and     -   (e) removing or adding any protecting groups if desired, and         forming a salt if desired.

Referring to method (a), examples of coupling reagents include any known coupling reagent, for examples peptide coupling reagents such as CDI (carbonyl diimidazole), DCC (N,N′-dicyclohexylcarbodiimide), and EDCI (1-(3-dimethylaminopropyl)-3-ethylcarboiimide). The reaction is optionally performed in the presence of an amine base, such as DIEA (diisopropylethylamine). Suitable solvents include dichloromethane, dichloroethane, THF, and DMF. The reaction is conveniently performed at ambient temperature.

Compounds of formula II

can be prepared by reducing a corresponding compound of formula III

under standard reducing conditions, for example reacting a compound of formula II with zinc dust under acidic conditions, such as in the presence of an acid such as NH₄Cl.

Compounds of Formula III can be prepared by coupling a corresponding compound having the formula IV

where Z is a leaving atom or group such as a halogen (for example Cl), with a corresponding compound having the formula V

where R³, R⁴, n, X and Y are as defined herein, in a suitable solvent such as an alcohol (for example n-butanol or isoproanol), at elevated temperatures, for example at temperatures between 100 and 180° C., for example at a temperature of about 140° C.

Compounds of the formula IV can be prepared from a corresponding compound of Formula V

using standard nitrating conditions known in the art, for example by reacting a corresponding compound of Formula V with nitric acid in the presence of an activating agent such as TFA or concentrated sulfuric acid. Compounds of Formula V are commercially available or can be prepared by standard methods known in the art.

Compounds of Formula II and III are also believed to be novel and provide a further embodiment of this invention.

Referring to method (b), suitable solvents include dichloromethane, dichloroethane, THF, and DMF. The reaction is conveniently performed at ambient temperature.

Referring to method (c), suitable coupling reagents include HATU, HBTU and other coupling reagents well known to persons skilled in the art. Suitable bases include amine bases such as diisopropylethylamine (DIEA) and triethylamine. Suitable solvents include DMF and CH₃CN. The reaction is conveniently performed at temperatures between 0° C. and ambient temperature.

Referring to method (d), suitable bases include amine bases such as pyridine or triethylamine. Suitable solvents include dichloromethane and dichloroethane. The reaction is conveniently performed at temperatures between 0° C. and ambient temperature.

Referring to method (e), suitable bases include amine bases (for example DIEA or triethylamine) and alkali metal carbonate bases (for example, potassium carbonate or sodium carbonate). In certain embodiments, compounds of formula II are treated with an amine base, and subsequently the chloroformate compound is added followed by the addition of the alkali metal carbonate base. Suitable solvents include DCM, DCE and THF. The reaction is conveniently performed at ambient temperature.

The ability of compounds to act as Trk-A inhibitors may be demonstrated by the assays described in Examples A and B. The ability of compounds to act as Trk-A inhibitors may be demonstrated by the assay described in Example B.

Compounds of Formula I are useful for treating chronic and acute pain, including pain associated with cancer. Certain compounds which are inhibitors of TrkA and/or TrkB may be useful in the treatment of multiple types of pain including inflammatory pain, neuropathic pain, and pain associated with cancer, surgery, and bone fracture.

Compounds of Formula I are also useful for treating cancers including neuroblastoma, ovarian, pancreatic and colorectal cancer.

Compounds of Formula I are also useful for treating inflammation and certain infectious diseases.

In addition, compounds of Formula I may also be used to treat interstitial cystitis (IC), painful bladder syndrome (PBS), urinary incontinence, asthma, anorexia, atopic dermatitis, and psoriasis.

Compounds of Formula I may also be used to treat demyelination and dysmyelination by promoting myelination, neuronal survival, and oligodendrocyte differentiation via blocking Sp35-TrkA interaction.

Compounds of Formula I may be of therapeutic value for the useful in the treatment of bone-related diseases (such as those involving bone resorption). Examples of bone-related diseases include metastatic bone disease, treatment-induced bone loss, osteoporosis, rheumatoid arthritis, ankylosing spondylitis, Paget's disease, and periodontal disease. The osteoporosis may be attributed to (1) menopause in women, (2) aging in men or women, (3) suboptimal bone growth during childhood and adolescence that resulted in failure to reach peak bone mass, and/or (4) bone loss secondary to other disease conditions, eating disorders, medications and/or medical treatments.

Other osteolytic diseases that can be treated according to the present invention are more localized. A particular example is metastatic tumor-induced osteolysis. In this condition, bone cancers or bone metastases induce localized osteolysis that causes pain, bone weakness and fractures. Such localized osteolysis also permits tumors to grow larger by creating more space for them in the bone and releasing growth factors from the bone matrix. Cancers presently known to cause tumor-induced osteolysis include hematological malignancies (e.g., myeloma and lymphoma) and solid tumors (e.g., breast, prostate, lung, renal and thyroid), all of which the present invention contemplates treating.

As used herein, the term treatment includes prophylaxis as well as treatment of an existing condition.

Accordingly, another aspect of this invention provides a method of treating diseases or medical conditions in a mammal, wherein said disease or condition is treatable with an inhibitor or Trk-A and/or Trk-B, comprising administering to said mammal one or more compounds of Formula I or a pharmaceutically acceptable salt thereof in an amount effective to treat or prevent said disorder. In a particular embodiment, the invention provides a method of treating pain, cancer, inflammation, neurodegenerative disease or Typanosoma cruzi infection in a mammal, which comprises administering to said mammal a therapeutically effective amount of a compound of Formula I or a pharmaceutically acceptable salt thereof. In another embodiment, the invention provides a method of treating osteolytic disease in a mammal, which comprises administering to said mammal a therapeutically effective amount of a compound of Formula I or a pharmaceutically acceptable salt thereof.

The compounds of the present invention can be used in combination with one or more additional drugs that work by the same or a different mechanism of action. Examples include anti-inflammatory compounds, steroids (e.g., dexamethasone, cortisone and fluticasone), analgesics such as NSAIDs (e.g., aspirin, ibuprofen, indomethacin, and ketoprofen), and opioids (such as morphine), and chemotherapeutic agents.

The phrase “effective amount” means an amount of compound that, when administered to a mammal in need of such treatment, is sufficient to (i) treat or prevent a particular disease, condition, or disorder which can be treated with an inhibitor or Trk-A and/or Trk-B, (ii) attenuate, ameliorate, or eliminate one or more symptoms of the particular disease, condition, or disorder, or (iii) prevent or delay the onset of one or more symptoms of the particular disease, condition, or disorder described herein.

The amount of a compound of Formula I that will correspond to such an amount will vary depending upon factors such as the particular compound, disease condition and its severity, the identity (e.g., weight) of the mammal in need of treatment, but can nevertheless be routinely determined by one skilled in the art.

As used herein, the term “mammal” refers to a warm-blooded animal that has or is at risk of developing a disease described herein and includes, but is not limited to, guinea pigs, dogs, cats, rats, mice, hamsters, and primates, including humans.

Compounds of the invention may be administered by any convenient route, e.g. into the gastrointestinal tract (e.g. rectally or orally), the nose, lungs, musculature or vasculature, or transdermally or dermally. Compounds may be administered in any convenient administrative form, e.g. tablets, powders, capsules, solutions, dispersions, suspensions, syrups, sprays, suppositories, gels, emulsions, patches etc. Such compositions may contain components conventional in pharmaceutical preparations, e.g. diluents, carriers, pH modifiers, sweeteners, bulking agents, and further active agents. If parenteral administration is desired, the compositions will be sterile and in a solution or suspension form suitable for injection or infusion. Such compositions form a further aspect of the invention.

According to another aspect, the present invention provides a pharmaceutical composition, which comprises a compound of Formula I or a pharmaceutically acceptable salt thereof, as defined hereinabove. In one embodiment, the pharmaceutical composition includes the compound of Formula I together with a pharmaceutically acceptable diluent or carrier.

According to another aspect, the present invention provides a compound of Formula I or a pharmaceutically acceptable salt thereof, for use in therapy, such as the treatment of a condition treatable with an inhibitor or Trk-A and/or Trk-B, such as one or more conditions described herein.

According to a further aspect, the present invention provides the use of a compound of Formula I or a pharmaceutically acceptable salt thereof, in the treatment of a condition that can be treated with an inhibitor or Trk-A and/or Trk-B, such as a condition as defined hereinabove. In one embodiment, the invention provides a compound of Formula I, or a pharmaceutically acceptable salt thereof, for use in the treatment of pain, cancer, inflammation, neurodegenerative disease or Typanosoma cruzi infection.

In one embodiment, a compound of the invention is selected from any one of:

-   (R)-1-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-phenylurea; -   (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)morpholine-4-carboxamide; -   (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)acetamide; -   (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-4-(methylsulfonamido)benzamide; -   (R)-1-(3-cyanophenyl)-3-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)urea; -   (R)-1-(4-cyanophenyl)-3-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)urea; -   (R)-1-(2,4-dichlorophenyl)-3-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)urea; -   (R)-1-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-(3-(trifluoromethyl)phenyl)urea; -   (R)-1-(3,5-dichlorophenyl)-3-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)urea; -   (S)—N-(6-((R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-hydroxypyrrolidine-1-carboxamide; -   (R)—N-(6-((R)-2-(2,5-di     fluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-hydroxypyrrolidine-1-carboxamide; -   (R)-tert-butyl     1-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-ylcarbamoyl)piperidin-4-ylcarbamate; -   (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-hydroxyazetidine-1-carboxamide; -   (R)-tert-butyl     1-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-ylcarbamoyl)azetidin-3-ylcarbamate; -   (R)-tert-butyl     4-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-ylcarbamoyl)piperazine-1-carboxylate; -   (R)-3-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-1,1-dimethylurea; -   tert-Butyl     1-(6-((R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-ylcarbamoyl)piperidin-3-ylcarbamate; -   (R)-4-amino-N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)piperidine-1-carboxamide; -   (R)-3-amino-N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)azetidine-1-carboxamide; -   (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)piperazine-1-carboxamide; -   3-Amino-N-(6-((R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)piperidine-1-carboxamide;

(R)-1-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-(4-fluorophenyl)urea;

-   tert-Butyl     3-(6-((R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-ylcarbamoyl)-3,8-diazabicyclo[3.2.1]octane-8-carboxylate; -   N-(6-((R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3,8-diazabicyclo[3.2.1]octane-3-carboxamide; -   (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-4-hydroxypiperidine-1-carboxamide; -   (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)piperidine-1-carboxamide; -   (R)-1-(6-(2-(2,5-di     fluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)urea; -   (R)-1-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-methylurea; -   (R)-1-tert-butyl-3-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)urea; -   (R)-1-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-(4-methoxyphenyl)urea; -   (R)-ethyl     1-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-ylcarbamoyl)piperidine-4-carboxylate; -   (R)-1-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-(3,4,5-trimethoxyphenyl)urea; -   (R)-1-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-(3,5-dimethylisoxazol-4-yl)urea; -   (R)-1-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-ylcarbamoyl)piperidine-4-carboxylic     acid; -   N-(6-((R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3,5-dimethylpiperazine-1-carboxamide; -   (R)-tert-butyl     4-(6-((R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-ylcarbamoyl)-2-methylpiperazine-1-carboxylate; -   (S)-tert-butyl     4-(6-((R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-ylcarbamoyl)-2-methylpiperazine-1-carboxylate; -   (R)—N-(6-((R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-methylpiperazine-1-carboxamide; -   (S)—N-(6-((R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-methylpiperazine-1-carboxamide; -   (3R,4R)—N-(6-((R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3,4-dihydroxypyrrolidine-1-carboxamide; -   (R)-1-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-ylcarbamoyl)piperidin-4-sulfone; -   (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-oxopiperazine-1-carboxamide; -   N-(6-(2-(3-fluorophenyl)piperidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-hydroxyazetidine-1-carboxamide; -   (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3,3-difluoropyrrolidine-1-carboxamide; -   (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3,3-difluoroazetidine-1-carboxamide; -   (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)azetidine-1-carboxamide; -   (R)-3-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-1-(2-hydroxyethyl)-1-methylurea; -   (R)-tert-butyl     4-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-ylcarbamoyl)-2,2-dimethylpiperazine-1-carboxylate; -   (S)-tert-butyl     4-(6-((R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-ylcarbamoyl)-2-isopropylpiperazine-1-carboxylate; -   (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3,3-dimethylpiperazine-1-carboxamide; -   (S)—N-(6-((R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-isopropylpiperazine-1-carboxamide; -   (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-(hydroxymethyl)azetidine-1-carboxamide; -   (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-methoxyazetidine-1-carboxamide; -   (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-hydroxy-3-methylazetidine-1-carboxamide; -   (R)-1-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-ylcarbamoyl)azetidin-3-yl     isobutyrate; -   (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-1-methyl-6-oxo-1,6-dihydropyridine-3-carboxamide; -   (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-1-methyl-6-oxo-1,6-dihydropyridazine-3-carboxamide; -   (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-4-methylpiperazine-1-carboxamide; -   (R)—N-(6-(2-(2,5-di     fluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-4-hydroxy-4-(trifluoromethyl)piperidine-1-carboxamide; -   (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-2,2,2-trifluoroacetamide;

and salts thereof. Particular examples of salts include hydrochloride and trifluoroacetate salts.

EXAMPLES

The following examples illustrate the invention. In the examples described below, unless otherwise indicated all temperatures are set forth in degrees Celsius. Reagents were purchased from commercial suppliers such as Aldrich Chemical Company, Lancaster, TCI or Maybridge, and were used without further purification unless otherwise indicated. Tetrahydrofuran (THF), dichloromethane (DCM, methylene chloride), toluene, and dioxane were purchased from Aldrich in Sure seal bottles and used as received.

The reactions set forth below were done generally under a positive pressure of nitrogen or argon or with a drying tube (unless otherwise stated) in anhydrous solvents, and the reaction flasks were typically fitted with rubber septa for the introduction of substrates and reagents via syringe. Glassware was oven dried and/or heat dried.

Column chromatography was done on a Biotage system (Manufacturer: Dyax Corporation) having a silica gel column or on a silica SepPak cartridge (Waters).

Abbreviations used in the Examples have the following meanings:

CDI: carbonyl diimidazole

HATU: 2(1H-azabenzotriazol-1-yl)-1,1,3,3-tetramethyl uranium hexafluorophophate methanaminium

DIEA: diisopropylethylamine

DMF: dimethylformamide

MTBE: methyl t-butyl ether

TFA: trifluoroacetic acid

ACN: acetonitrile

IPA: isopropyl alcohol

Example A TrkA ELISA Assay

An enzyme-linked immunosorbant assay (ELISA) was used to assess TrkA kinase activity in the presence of inhibitors. Immulon 4HBX 384-well microtiter plates (Thermo part #8755) were coated with a 0.025 mg/mL solution of poly (Glu, Ala, Tyr; 6:3:1; Sigma P3899). Various concentrations of test compound, 2.5 nM TrkA (Invitrogen Corp., histidine-tagged recombinant human TrkA, cytoplasmic domain), and 500 μM ATP were incubated for 25 minutes at ambient temperature in the coated plates while shaking. The assay buffer consisted of 25 mM MOPS pH 7.5, 0.005% (v/v) Triton X-100 and 5 mM MgCl₂. The reaction mixture was removed from the plate by washing with PBS containing 0.1% (v/v) Tween 20. The phosphorylated reaction product was detected using 0.2 μg/mL of a phosphotyrosine specific monoclonal antibody (clone PY20) conjugated to horseradish peroxidase in conjunction with the TMB Peroxidase Substrate System (KPL). After the addition of 1M phosphoric acid, the chromogenic substrate color intensity was quantitated via absorbance at 450 nm. IC₅₀ values were calculated using either a 4 or 5-parameter logistic curve fit.

In this assay, compounds of the invention had an average IC₅₀ below 1000 nM. Certain compounds had an average IC₅₀ below 100 nM.

Table 1 provides IC₅₀ values for compounds of the invention when tested in this assay.

TABLE 1 TrkA Elisa Enzyme Example # IC₅₀ (nM) 1 8.3 2 23.7 3 5.4 4 2.1 5 74.2 6 10.7 7 39.4 8 507.8 9 716.7 10 3.8 11 15.5 12 17.2 13 9.4 14 23.2 15 33.6 16 18 17 13.8 18 52.9 19 126.3 20 94.7 21 42 22 10 23 75.5 24 107.1 25 13.8 26 7.1 27 77.1 28 65.7 29 9.8 30 5.5 31 20.1 32 175.6 33 901 34 64.4 35 49.6 36 13 37 40.6 38 47.9 39 29.9 40 2.2 41 884.4 42 26.2 43 215.6 44 22.7 45 92 46 17.9 47 10.3 48 8.3 49 857 50 60.6 51 27.7 52 14 53 16.4 54 8.9 55 19.4 56 10.2 57 2.3 58 53.2 59 16.5 60 22

Example B TrkA and TrkB Omnia Assay

Trk enzymatic selectivity was assessed using Omnia™ Kinase Assay reagents from Invitrogen Corp. Enzyme (either TrkA or TrkB from Invitrogen Corp.) and test compound (various concentrations) were incubated for 10 minutes at ambient temperature in a 384-well white polypropylene plate (Nunc catalog #267462). Omnia Tyr Peptide #4 (for TrkA) or #5 (for TrkB), as well as ATP, were then added to the plate. Final concentrations were as follows: 20 nM enzyme, 500 μM of ATP for TrkA assay or 1 mM ATP for TrkB assay, 10 μM peptide substrate. The assay buffer consisted of 25 mM MOPS pH 7.5, 0.005% (v/v) Triton X-100 and 5 mM MgCl₂. The production of phosphorylated peptide was monitored continuously for 70 minutes using a Molecular Devices FlexStation II³⁸⁴ microplate reader (excitation=360 nm; emission=485 nm). Initial rates were calculated from the progress curves. IC₅₀ values were then calculated from these rates using either a 4 or 5-parameter logistic curve fit.

In this assay, compounds of the invention had an average IC₅₀ below 1000 nM. Certain compounds had an average IC₅₀ below 100 nM.

Preparation A

Preparation of (R)-2-(2,5-difluorophenyl)pyrrolidine Step A: Preparation of (R)-tert-butyl 2-(2,5-difluorophenyl)pyrrolidine-1-carboxylate

A solution of tert-butyl pyrrolidine-1-carboxylate (20 g, 116.8 mmol) and (−) sparteine (32.9, 140 mmol) in MTBE (360 mL) was cooled to −78° C., and sec-BuLi (100 mL, 140 mmol, 1.4 M in cyclohexane) was introduced drop-wise via cannula, keeping the internal temperature under −70° C. The resulting solution was stirred for 3 hours at −78° C., followed by addition of a solution of ZnCl₂ (93.4 mL, 93.4 mmol, 1M in Et₂O) drop-wise with rapid stirring, keeping the internal temperature below −65° C. The resulting light suspension was stirred at −78° C. for 30 minutes and then warmed to ambient temperature. The resulting mixture was charged with 2-bromo-1,4-difluorobenzene (14.5 mL, 128 mmol), followed by Pd(OAc)₂ (1.31 g, 5.8 mmol) and t-Bu₃P—HBF₄ (2.03 g, 7.0 mmol) in one portion. After stirring overnight at ambient temperature, 10.5 mL of NH₄OH solution was added and the reaction was stirred for another hour. The resulting slurry was filtered through CELITE and washed with Et₂O (1 L). The filtrate was washed with HCl (0.5 L, 1M aq.) and brine. The organic layer was filtered and concentrated, and the crude product was purified by silica column chromatography, eluting with 5-10% EtOAc/hexanes to give product (R)-tert-butyl 2-(2,5-difluorophenyl)pyrrolidine-1-carboxylate as yellow oil (23.9 g, 72% yield).

Step B: Preparation of (R)-2-(2,5-difluorophenyl)pyrrolidine

To (R)-tert-butyl 2-(2,5-difluorophenyl)pyrrolidine-1-carboxylate (23.9 g, 84.4 mmol) was added 56.2 mL 4N HCl (dioxane). After stirring at ambient temperature for 2 hours, 200 mL of ether was added and the mixture was stirred for 10 minutes. The resulting slurry was filtered, yielding the hydrochloride salt of the product as a white solid (17.2 g). To obtain the free base, the HCl salt product was dispersed in a mixture of EtOAc (200 mL) and NaOH solution (100 mL, 2 N aq.) The layers were separated and the aqueous layer was extracted with EtOAc. The combined organic extracts were filtered and concentrated to give the desired product as a liquid (13.2 g, 85% yield).

The Enantiomeric Excess (ee %) of (R)-2-(2,5-difluorophenyl)pyrrolidine was determined as follows

To an ethanol solution of (R)-2-(2,5-difluorophenyl)pyrrolidine was added excess N-(2,4-dinitro-5-fluorophenyl)-L-alanine amide (FDAA, Marfey's reagent). The mixture was heated to reflux for approximately two minutes. After cooling to ambient temperature, the reaction mixture was diluted with acetonitrile and injected onto HPLC (YMC ODS-AQ 4.6×50 mm 3 μm 120 Å column; mobile phase: 5-95% solvent B in A; solvent A: H₂O/1% IPA/10 mM ammonium acetate, and solvent B: ACN/1% IPA/10 mM ammonium acetate; flow rate: 2 mL/min) to determine the enantiomeric excess of the product by calculating the peak areas of the two diastereomeric derivatives formed. A 1:1 racemic sample was prepared according the same procedure described herein, replacing (R)-2-(2,5-difluorophenyl)pyrrolidine with (rac)-2-(2,5-difluorophenyl)pyrrolidine. The ee % of the product obtained as described above was determined to be >93%.

Preparation B

Preparation of (R)-6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-amine

Step 1: Preparation of 6-chloro-3-nitroimidazo[1,2-b]pyridazine

6-Chloroimidazo[1,2-b]pyridazine (4.95 g, 31.6 mmol) [purchased from Combi-Blocks] was dissolved in 60 mL concentrated sulfuric acid, cooled in an ice bath, and nitric acid (9.9 mL, 158 mmol) was added dropwise while stirring. The reaction was stirred at 0° C. for 30 minutes, then at ambient temperature for 4.5 hours to reach completion. The reaction was poured onto ice, and the resulting aqueous mixture was neutralized with 50% NaOH aqueous solution and then extracted with EtOAc (3×400 mL). The organic layers were combined and washed with water (2×400 mL) and brine (400 mL), dried (Na₂SO₄), filtered and concentrated to yield the product as a yellowish powder (5.7 g, 91% yield).

Step 2: Preparation of (R)-6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)-3-nitro imidazo[1,2-b]pyridazine

A suspension of 6-chloro-3-nitro imidazo[1,2-b]pyridazine (1.0 g, 5.0 mmol) and (R)-2-(2,5-difluorophenyl)pyrrolidine (Prepared as described in Preparation A; 1.9 g, 11 mmol) in n-butanol (4.6 mL, 50 mmol) was sealed in a pressure reaction tube and stirred in a 140° C. oil bath overnight. After cooling to ambient temperature, the reaction mixture was diluted with EtOAc (250 mL), then washed with water (2×150 mL) and brine (150 mL), filtered through a Biotage Phase Separator filter paper and concentrated. The crude material was purified by silica gel chromatography, eluting with 2:1 EtOAc/hexanes to yield the product as a foamy yellow powder (1.3 g, 75% yield).

Step 3: Preparation of (R)-6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-amine

To a mixture of (R)-6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)-3-nitroimidazo[1,2-b]pyridazine (4.17 g, 12.1 mmol) and SnCl₂ dihydrate (10.9 g, 48.4 mmol) in a flask was added 200 mL EtOH to form a suspension. The reaction was heated at 70° C. for 1 hour to reach completion. After cooling to ambient temperature, the reaction mixture was concentrated. Water (200 mL) was added to the resulting crude solid residue, and the mixture was briefly sonicated and then vacuum-filtered. The filtrate pH was neutralized with 6N NaOH solution and extracted with DCM (3×250 mL). The combined organic layers were washed with brine (200 mL), dried over Na₂SO₄, and concentrated to yield the crude product as a yellowish foamy solid. The crude material was purified by C-18 reverse-phase column chromatography (eluent=5 to 60% acetonitrile/water) to provide the pure product as a light yellowish powder (3 g, 78% yield).

Example 1

Preparation of (R)-1-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-phenylurea

To a DCM (0.1 mL) solution of (R)-6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-amine (Preparation B; 6 mg, 0.019 mmol) cooled in an ice bath was added the isocyanatobenzene (2.5 mg, 0.021 mmol) in DCM (0.1 mL) dropwise. The reaction was slowly warmed to ambient temperature and stirred for 1 hour. The reaction was diluted with DCM (2 mL), washed with water, and concentrated. The crude product was purified by silica gel chromatography (eluent=50% EtOAc/hexanes first, then 5% MeOH in DCM) to yield the pure final product as a solid (5 mg, 60%). MS (apci) m/z=435.2 (M+H).

Example 2

(R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)morpholine-4-carboxamide

To a DCM (1.9 mL) solution of (R)-6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-amine (Preparation B; 72 mg, 0.19 mmol) was added 1,1′-carbonyldiimidazole (CDI) (47 mg, 0.29 mmol) at ambient temperature in one portion. After stirring for 2 hours, morpholine (34 mg, 0.39 mmol) was added in one portion. The reaction was stirred for another hour before it was concentrated, then directly purified by reverse-phase column chromatography, eluting with 5 to 60% acetonitrile/water to yield (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)morpholine-4-carboxamide as a solid (64 mg, 77% yield). MS (apci) m/z=429.1 (M+H).

Example 3

(R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)acetamide

To a DCM (0.1 mL) solution of (R)-6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-amine (Preparation B; 6 mg, 0.019 mmol) cooled in an ice bath was added acetic anhydride (2.1 mg, 0.021 mmol), followed by pyridine (2 mg, 0.025 mmol). The reaction was warmed to ambient temperature and stirred for 1 hour before it was concentrated and directly purified by reverse-phase column chromatography, eluting with 5 to 60% acetonitrile/water to yield (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)acetamide as an off-white solid (6 mg, 81% yield). MS (apci) m/z=358.2 (M+H).

Example 4

(R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-4-(methylsulfonamido)benzamide

A reaction vial was charged with (R)-6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-amine (Preparation B; 30 mg, 0.095 mmol), 4-(methylsulfonamido)benzoic acid (41 mg, 0.19 mmol), and 2(1H-azabenzotriazol-1-yl)-1,1,3,3-tetramethyl uranium hexafluorophophate methanaminium (HATU; 72 mg, 0.19 mmol). DMF (0.8 mL) was added to the mixture to make a solution. The reaction mixture was cooled in an ice bath for 10 minutes before DIEA (0.05 mL, 0.29 mmol) was added dropwise. After addition, reaction was warmed to ambient temperature and stirred overnight. The reaction mixture was diluted with EtOAc (20 mL), washed with water and brine (10 mL each), and concentrated. The crude material was purified by reverse-phase column chromatography, eluting with 5 to 60% acetonitrile/water to yield (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-4-(methylsulfonamido)benzamide as a yellowish solid (13 mg, 27% yield). MS (apci negative) m/z=511.4 (M−H).

Example 5

(R)-1-(3-cyanophenyl)-3-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)urea

To a DCM (0.1 mL) solution of (R)-6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-amine (Preparation B; 6 mg, 0.019 mmol) cooled in an ice bath was added 3-cyanophenylisocyanate (14 mg, 0.095 mmol) in DCM (0.1 mL) drop-wise. The reaction was slowly warmed to ambient temperature and stirred for 1 hour. The reaction was diluted with DCM (2 mL), washed with water, and concentrated. The crude material was purified by reverse-phase column chromatography, eluting with 5 to 85% acetonitrile/water to yield the pure final product as a solid (3.2 mg, 37% yield). MS (apci) m/z=460.2 (M+H).

Example 6

(R)-1-(4-cyanophenyl)-3-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1, 2-b]pyridazin-3-yl)urea

Prepared according to Example 5, replacing 3-cyanophenylisocyanate with 4-cyanophenylisocyanate to yield the final product as a solid. MS (apci) m/z=460.2 (M+H).

Example 7

(R)-1-(2,4-dichlorophenyl)-3-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)urea

Prepared according to Example 5, replacing 3-cyanophenylisocyanate with 2,4-dichlorophenylisocyanate to yield the final product as a solid. MS (apci) m/z=503.1, 505.1 (M+H, M+3H).

Example 8

(R)-1-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-(3-(trifluoromethyl)phenyl)urea

Prepared according to Example 5, replacing 3-cyanophenylisocyanate with 3-trifluoromethylphenylisocyanate to yield the final product as a solid (6.5 mg, 68% yield). MS (apci) m/z=503.2 (M+H).

Example 9

(R)-1-(3,5-dichlorophenyl)-3-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)urea

Prepared according to Example 5, replacing 3-cyanophenylisocyanate with 3,5-dichlorophenylisocyanate to yield the final product as a solid. MS (apci) m/z=503.1 (M+H), 505.1 (M+3H).

Example 10

(S)—N-(6-((R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-hydroxypyrrolidine-1-carboxamide

Prepared according to the method of Example 2, replacing morpholine with (S)-pyrrolidin-3-ol [purchased from SUVEN Life Sciences] to yield the final product as a solid (79 mg, 68% yield). MS (apci) m/z=429.2 (M+H).

Example 11

(R)—N-(6-((R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-hydroxypyrrolidine-1-carboxamide

Prepared according to the method of Example 2, replacing morpholine with (R)-pyrrolidin-3-ol to yield the final product as a solid (8 mg, 77% yield). MS (apci) m/z=429.2 (M+H).

Example 11A

(R)—N-(6-((R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-hydroxypyrrolidine-1-carboxamide hydrochloride

To a methanol (1 mL) solution of (S)—N-(6-((R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-hydroxypyrrolidine-1-carboxamide (10.1 mg, 0.0236 mmol) was added HCl as a solution is dioxane (30 μL). After minutes, the reaction was concentrated to provide (S)—N-(6-((R)-2-(2,5-di fluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-hydroxypyrrolidine-1-carboxamide hydrochloride as a yellow solid.

Example 12

(R)-tert-butyl 1-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-ylcarbamoyl)piperidin-4-ylcarbamate

Prepared according to the method of Example 2, replacing morpholine with tert-butyl piperidin-4-ylcarbamate to yield the final product as a solid (10 mg, 76% yield). MS (apci) m/z=542.2 (M+H).

Example 13

(R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-hydroxyazetidine-1-carboxamide

To a DCM (1 mL) solution of (R)-6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-amine (Preparation B; 50 mg, 0.16 mmol) was added CDI (39 mg, 0.24 mmol) at ambient temperature in one portion while stirring. After 1 hour stirring, azetidin-3-ol hydrochloride (35 mg, 0.32 mmol) [purchased from Oakwood] was added in one portion, followed by addition of DIEA (83 μL, 0.48 mmol). The reaction mixture was briefly sonicated to help break up the solid particles from azetidine material. After 30 minute stirring at ambient temperature, the reaction was concentrated and directly purified by reverse-phase column chromatography, eluting with 5 to 50% acetonitrile/water to yield the final product as a pale-yellowish solid (65 mg, 99% yield). MS (apci) m/z=415.2 (M+H).

Example 14

(R)-tert-butyl 1-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-ylcarbamoyl)azetidin-3-ylcarbamate

Prepared according to the method of Example 2, replacing morpholine with tert-butyl azetidin-3-ylcarbamate to yield the final product as a solid (10 mg, 80% yield). MS (apci) m/z=514.2 (M+H).

Example 15

(R)-tert-butyl 4-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-ylcarbamoyl)piperazine-1-carboxylate

Prepared according to the method of Example 2, replacing morpholine with tert-butyl piperazine-1-carboxylate to yield the final product as a solid (10 mg, 78% yield). MS (apci) m/z=528.2 (M+H).

Example 16

(R)-3-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-1,1-dimethylurea

Prepared according to the method of Example 2, replacing morpholine with dimethylamine to yield the final product as a solid (8 mg, 85% yield). MS (apci) m/z=387.2 (M+H).

Example 17

tert-Butyl 1-(6-((R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-ylcarbamoyl)piperidin-3-ylcarbamate

Prepared according to the method of Example 2, replacing morpholine with tert-butyl piperidin-3-ylcarbamate to yield the final product as a solid (10 mg, 76% yield). MS (apci) m/z=542.3 (M+H).

Example 18

(R)-4-amino-N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)piperidine-1-carboxamide

(Example 12, 10 mg, 0.018 mmol) was dissolved in 0.2 mL DCM, followed by addition of 0.5 mL 4 N HCl (dioxane) solution in one portion. After stirring at ambient temperature overnight, the reaction was concentrated to yield the final product salt form as a light yellowish solid. MS (apci) m/z=442.1 (M+H).

Example 19

(R)-3-amino-N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)azetidine-1-carboxamide trifluoroacetate

(R)-tert-butyl 1-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-ylcarbamoyl)azetidin-3-ylcarbamate (Example 14; 10 mg, 0.019 mmol) was dissolved in 0.5 mL 50% TFA in DCM and stirred at ambient temperature for 2 hours. The reaction is concentrated, treated with ether, and concentrated again to yield the final product salt form as a white solid. MS (apci) m/z=414.2 (M+H).

Example 20

(R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)piperazine-1-carboxamide hydrochloride

(R)-tert-butyl 4-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-ylcarbamoyl)piperazine-1-carboxylate (Example 15; 10 mg, 0.019 mmol) was dissolved in 0.2 mL DCM, followed by addition of 0.5 mL 4 N HCl (dioxane) solution in one portion. After stirring at ambient temperature overnight, the reaction was concentrated to yield the final product salt form as a light yellowish solid. MS (apci) m/z=428.2 (M+H).

Example 21

3-Amino-N-(6-((R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)piperidine-1-carboxamide hydrochloride

tert-Butyl 1-(6-((R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-ylcarbamoyl)piperidin-3-ylcarbamate (Example 17; 10 mg, 0.018 mmol) was dissolved in 0.1 mL DCM, followed by addition of 0.5 mL 4 N HCl (dioxane) solution in one portion. After stirring at ambient temperature overnight, the reaction was concentrated to yield the final product salt form as a light yellowish solid. MS (apci) m/z=442.1 (M+H).

Example 22

(R)-1-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-(4-fluorophenyl)urea

Prepared according to Example 5, replacing 3-cyanophenylisocyanate with 1-fluoro-4-isocyanatobenzene to yield the final product as a solid. MS (apci) m/z=453.2 (M+H).

Example 23

tert-Butyl 3-(6-((R)-2-(2,5-di fluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-ylcarbamoyl)-3,8-diazabicyclo[3.2.1]octane-8-carboxylate

Prepared according to the method of Example 2, replacing morpholine with tert-butyl 3,8-diazabicyclo[3.2.1]octane-8-carboxylate to yield the final product as a solid. MS (apci) m/z=554.2 (M+H).

Example 24

N-(6-((R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3,8-diazabicyclo[3.2.1]octane-3-carboxamide hydrochloride

tert-Butyl 3-(6-((R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-ylcarbamoyl)-3,8-diazabicyclo[3.2.1]octane-8-carboxylate (Example 23, 10 mg, 0.018 mmol) was dissolved in 0.1 mL DCM, followed by addition of 0.5 mL 4 N HCl (dioxane) solution in one portion. After stirring at ambient temperature overnight, the reaction was concentrated to yield the final product salt form as a light yellowish solid. MS (apci) m/z=454.1 (M+H).

Example 25

(R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-4-hydroxypiperidine-1-carboxamide

Prepared according to the method of Example 2, replacing morpholine with piperidin-4-ol to yield the final product as a solid. MS (apci) m/z=443.2 (M+H).

Example 26

(R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)piperidine-1-carboxamide

Prepared according to the method of Example 2, replacing morpholine with piperidine, to yield the final product as a solid. MS (apci) m/z=427.2 (M+H).

Example 26A

(R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)piperidine-1-carboxamide hydrochloride

To a methanol (1 mL) solution of (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)piperidine-1-carboxamide (4.9 mg, 0.011 mmol) was added HCl as a solution is dioxane (30 μL). After 30 minutes, the reaction was concentrated to provide (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)piperidine-1-carboxamide hydrochloride (4.2 mg, 0.0091 mmol, 79% yield) as a yellow solid. MS (apci) m/z=427.4 (M+H).

Example 27

(R)-1-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)urea

Prepared according to the method of Example 2, replacing morpholine with ammonia, to yield the final product as a solid. MS (apci) m/z=359.2 (M+H).

Example 28

(R)-1-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-methylurea

Prepared according to the method of Example 2, replacing morpholine with methylamine, to yield the final product as a solid. MS (apci) m/z=373.2 (M+H).

Example 29

(R)-1-tert-butyl-3-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)urea

Prepared according to Example 5, replacing 3-cyanophenylisocyanate with 2-isocyanato-2-methylpropane, to yield the final product as a solid. MS (apci) m/z=415.2 (M+H).

Example 30

(R)-1-(6-(2-(2,5-di fluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-(4-methoxyphenyl)urea

Prepared according to Example 5, replacing 3-cyanophenylisocyanate with 1-isocyanato-4-methoxybenzene to yield the final product as a solid (7.5 mg, 85% yield). MS (apci) m/z=465.2 (M+H).

Example 31

(R)-ethyl 1-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-ylcarbamoyl)piperidine-4-carboxylate

Prepared according to the method of Example 2, replacing morpholine with ethyl piperidine-4-carboxylate, to yield the final product as a solid. MS (apci) m/z=499.2 (M+H).

Example 32

(R)-1-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-(3,4,5-trimethoxyphenyl)urea

Prepared according to Example 5, replacing 3-cyanophenylisocyanate with 5-isocyanato-1,2,3-trimethoxybenzene to yield the final product as a solid (3.2 mg, 32% yield). MS (apci) m/z=525.2 (M+H).

Example 33

(R)-1-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-(3,5-dimethylisoxazol-4-yl)urea

Prepared according to Example 5, replacing 3-cyanophenylisocyanate with 4-isocyanato-3,5-dimethylisoxazole to yield the final product as a solid (8.1 mg, 94% yield). MS (apci) m/z=454.2 (M+H).

Example 34

(R)-1-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-ylcarbamoyl)piperidine-4-carboxylic acid

(R)-ethyl 1-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-ylcarbamoyl)piperidine-4-carboxylate (Example 31, 9.2 mg, 0.018 mmol) was dissolved in a mixture solvent of THF:MeOH:water (2:2:1 v/v; 0.2 mL), followed by addition of lithium hydroxide monohydrate (2.3 mg, 0.055 mmol). After stirring at ambient temperature overnight, the reaction was diluted with water (1 mL), acidified with 10% citric acid, and extracted with EtOAc (3×1 mL). The combined organic layers were concentrated, and the crude material was purified by reverse-phase column chromatography, eluting with 5 to 55% MeOH/water to yield the final product as a solid. MS (apci) m/z=471.2 (M+H).

Example 35

N-(6-((R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3,5-dimethylpiperazine-1-carboxamide

Prepared according to the method of Example 2, replacing morpholine with 2,6-dimethylpiperazine to yield the final product as a yellowish foamy powder (7.5 mg, 61% yield). MS (apci) m/z=456.2 (M+H).

Example 36

(R)-tert-butyl 4-(6-((R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-ylcarbamoyl)-2-methylpiperazine-1-carboxylate

Prepared according to the method of Example 2, replacing morpholine with (R)-tert-butyl 2-methylpiperazine-1-carboxylate, to yield the final product as an off-white foamy powder (12 mg, 82% yield). MS (apci) m/z=542.2 (M+H).

Example 37

(S)-tert-butyl 4-(6-((R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-ylcarbamoyl)-2-methylpiperazine-1-carboxylate

Prepared according to the method of Example 2, replacing morpholine with (S)-tert-butyl 2-methylpiperazine-1-carboxylate to yield the desired product as an off-white foamy powder (10 mg, 69% yield). MS (apci) m/z=542.2 (M+H).

Example 38

(R)—N-(6-((R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-methylpiperazine-1-carboxamide hydrochloride

To a reaction vial containing (R)-tert-butyl 4-(6-((R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-ylcarbamoyl)-2-methylpiperazine-1-carboxylate (Example 36; 12 mg, 0.022 mmol) was added 0.5 mL 4 N HCl (dioxane) solution in one portion. After stirring at ambient temperature for 4 hours, the reaction was concentrated. The resulting solid residue was treated with ether and concentrated again to yield the final product salt form as a pale-yellowish powder. MS (apci) m/z=442.2 (M+H).

Example 39

(S)—N-((4R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-methylpiperazine-1-carboxamide

Prepared according to the method of Example 38, replacing (R)-tert-butyl 4-(6-((R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-ylcarbamoyl)-2-methylpiperazine-1-carboxylate with (S)-tert-butyl 4-(6-((R)-2-(2,5-difluorophenyl) pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-ylcarbamoyl)-2-methylpiperazine-1-carboxylate (Example 37). The final product was a fine pale-yellowish powder. MS (apci) m/z=442.2 (M+H).

Example 40

(3R,4R)—N-(6-((R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3,4-dihydroxypyrrolidine-1-carboxamide

Prepared according to the method of Example 2, replacing morpholine with (3R,4R)-pyrrolidine-3,4-diol [obtained from benzyl de-protection of commercially available (3R,4R)-1-benzylpyrrolidine-3,4-diol] to yield the final product as a solid (11 mg, 92% yield). MS (apci) m/z=445.2 (M+H).

Example 41

(R)-1-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-ylcarbamoyl)piperidin-4-sulfone

Prepared according to the method of Example 2, replacing morpholine with piperidin-4-sulfone to yield the final product as a solid (10 mg, 78% yield). MS (apci) m/z=477.2 (M+H).

Example 42

(R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-oxopiperazine-1-carboxamide

Prepared according to the method of Example 2, replacing morpholine with piperazin-2-one to yield the final product as a solid (10 mg, 84% yield). MS (apci) m/z=442.1 (M+H).

Example 43

N-(6-(2-(3-fluorophenyl)piperidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-hydroxyazetidine-1-carboxamide Step 1: Preparation of 6-(2-(3-fluorophenyl)piperidin-1-yl)-3-nitroimidazo[1,2-b]pyridazine

To a pressure reaction tube were charged 6-chloro-3-nitroimidazo[1,2-b]pyridazine (450 mg, 2.27 mmol), 2-(3-fluorophenyl)piperidine (609 mg, 3.40 mmol, purchased from ChemBridge), and N-ethyl-N-isopropylpropan-2-amine (0.51 mL, 2.95 mmol), followed by addition of 1.0 mL n-butanol. The reaction mixture was then sealed and stirred at 180° C. for 24 hours. After completion, the reaction was cooled to ambient temperature, and diluted with water and EtOAc. The organic layer was separated, and the aqueous layer was extracted with EtOAc twice. The combined organic layers was dried over Na₂SO₄ and concentrated. The crude product was purified by silica column chromatography, eluting with 20 to 50% EtOAc in hexanes to yield the desired product for the next step.

Step 2: Preparation of 6-(2-(3-fluorophenyl)piperidin-1-yl)imidazo[1,2-b]pyridazin-3-amine

A mixture of 6-(2-(3-fluorophenyl)piperidin-1-yl)-3-nitroimidazo[1,2-b]pyridazine (50 mg, 0.146 mmol) and SnCl₂ dihydrate (165 mg, 0.732 mmol) in 5 mL EtOH was first stirred at 70° C. for 30 minutes, then cooled to ambient temperature and concentrated. EtOAc and water (10 mL each) were added to the solid residue, followed by Na₂CO₃ aqueous solution (2 mL×2 N) to obtain a phase break. The organic layer was separated, and the aqueous layer was extracted with EtOAc (3×10 mL). The combined organic layers was dried with Na₂SO₄ and concentrated to provide the product for the next step.

Step 3: Preparation of N-(6-(2-(3-fluorophenyl)piperidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-hydroxyazetidine-1-carboxamide

To a DCM (2 mL) solution of 6-(2-(3-fluorophenyl)piperidin-1-yl)imidazo[1,2-b]pyridazin-3-amine (45 mg, 0.14 mmol) was added CDI (35 mg, 0.22 mmol) at ambient temperature in one portion. After stirring for five hours, azetidin-3-ol hydrochloride (54 mg, 0.33 mmol) was added in one portion, followed by DIEA (0.05 mL, 0.29 mmol), and the reaction was stirred at ambient temperature overnight. The reaction was diluted with DCM, washed with water, dried over Na₂SO₄ and concentrated. The crude product was purified by reversed phase column, eluting with 0 to 55% CH₃CN/water to obtain the desired product as a solid (30 mg, 51% yield). MS (apci) m/z=411.2 (M+H).

Example 44

(R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3,3-difluoropyrrolidine-1-carboxamide

Prepared according to Example 13, replacing azetidin-3-ol hydrochloride with 3,3-difluoropyrrolidine hydrochloride to yield the final product as a white solid. MS (apci) m/z=449.2 (M+H).

Example 45

(R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3,3-difluoroazetidine-1-carboxamide

Prepared according to Example 13, replacing azetidin-3-ol hydrochloride with 3,3-difluoroazetidine hydrochloride to yield the final product as a solid (20 mg, 77% yield). MS (apci) m/z=435.2 (M+H).

Example 46

(R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)azetidine-1-carboxamide

Prepared according to Example 13, replacing azetidin-3-ol hydrochloride with azetidine to yield the final product as a solid (20 mg, 77% yield). MS (apci) m/z=399.2 (M+H).

Example 47

(R)-3-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-1-(2-hydroxyethyl)-1-methylurea

Prepared according to Example 13, replacing azetidin-3-ol hydrochloride with 2-(methylamino)ethanol to yield the final product as a solid (20 mg, 81% yield). MS (apci) m/z=417.2 (M+H).

Example 48

(R)-tert-butyl 4-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-ylcarbamoyl)-2,2-dimethylpiperazine-1-carboxylate

Prepared according to the method of Example 2, replacing morpholine with tert-butyl 2,2-dimethylpiperazine-1-carboxylate to yield the final product as a solid (40 mg, 91% yield). MS (apci) m/z=556.3 (M+H).

Example 49

(S)-tert-butyl 4-(6-((R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-ylcarbamoyl)-2-isopropylpiperazine-1-carboxylate

Prepared according to the method of Example 2, replacing morpholine with (S)-tert-butyl 2-isopropylpiperazine-1-carboxylate to yield the final product as a white foamy solid (42 mg, 93% yield). MS (apci) m/z=570.3 (M+H).

Example 50

(R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3,3-dimethylpiperazine-1-carboxamide trifluoroacetate

To a reaction vial containing (R)-tort-butyl 44642-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-ylcarbamoyl)-2,2-dimethylpiperazine-1-carboxylate (Example 48, 33.5 mg, 0.06 mmol) was added 1 mL TFA/DCM (1:1 v/v) and left at ambient temperature for 1 hour. After removal of solvent, the crude oil was treated with ether and gave the product TFA salt as a white solid. MS (apci) m/z=456.2 (M+H).

Example 51

(S)—N-(6-((R)-2-(2,5-di fluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-isopropylpiperazine-1-carboxamide

Prepared according to the method of Example 50, replacing (R)-tert-butyl 4-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-ylcarbamoyl)-2,2-dimethylpiperazine-1-carboxylate with (S)-tert-butyl 4-(6-((R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-ylcarbamoyl)-2-isopropylpiperazine-1-carboxylate (Example 49). The final product was a fine white solid. MS (apci) m/z=470.2 (M+H).

Example 52

(R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-(hydroxymethyl)azetidine-1-carboxamide

Prepared according to Example 13, replacing azetidin-3-ol hydrochloride with azetidin-3-ylmethanol hydrochloride to yield the final product as a pale-yellowish solid (18 mg, 53% yield). MS (apci) m/z=429.2 (M+H).

Example 52A

(R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-(hydroxymethyl)azetidine-1-carboxamide hydrochloride

To a methanol (1 mL) solution of (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-(hydroxymethyl)azetidine-1-carboxamide (9.9 mg, 0.0231 mmol) was added HCl as a solution is dioxane (30 μL). After 30 minutes, the reaction was concentrated to provide (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-(hydroxymethyl)azetidine-1-carboxamide hydrochloride (10.2 mg, 0.0219 mmol, 94.9% yield) as a yellow solid. MS (apci) m/z=429.4 (M+H).

Example 53

(R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-methoxyazetidine-1-carboxamide

Prepared according to Example 13, replacing azetidin-3-ol hydrochloride with 3-methoxyazetidine hydrochloride to yield the final product as a pale-yellowish solid (60 mg, 88% yield). MS (apci) m/z=429.2 (M+H).

Example 54

(R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-hydroxy-3-methylazetidine-1-carboxamide

Prepared according to Example 13, replacing azetidin-3-ol hydrochloride with 3-methylazetidin-3-ol hydrochloride to yield the final product as a pale-yellowish solid (63 mg, 93% yield). MS (apci) m/z=429.2 (M+H).

Example 54A

(R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-hydroxy-3-methylazetidine-1-carboxamide hydrochloride

To a methanol (1 mL) solution of (R)—N-(5-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)pyrazolo[1,5-a]pyrimidin-3-yl)-3-hydroxy-3-methylazetidine-1-carboxamide (10.2 mg, 0.0238 mmol) was added HCl as a solution is dioxane (30 μL). After 30 minutes, the reaction was concentrated to provide (R)—N-(5-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)pyrazolo[1,5-a]pyrimidin-3-yl)-3-hydroxy-3-methylazetidine-1-carboxamide hydrochloride (8.3 mg, 0.0179 mmol, 75.0% yield) as a yellow solid.

Example 55

(R)-1-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-ylcarbamoyl)azetidin-3-yl)isobutyrate

(R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-hydroxyazetidine-1-carboxamide (Example 13; 21.5 mg, 0.05 mmol) was first dissolved in 0.5 mL DMF, followed by addition of isobutyric anhydride (24 mg, 0.15 mmol) and a few drops of DIEA. After overnight stirring at ambient temperature, the crude material was concentrated and directly purified by silica chromatography, eluting with 3 to 8% MeOH in DCM to provide the final product as a beige foamy solid (12 mg, 50% yield). MS (apci) m/z=485.2 (M+H).

Example 56

(R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-1-methyl-6-oxo-1,6-dihydropyridine-3-carboxamide

Prepared according to Example 4, replacing 4-(methylsulfonamido)benzoic acid with 1-methyl-6-oxo-1,6-dihydropyridine-3-carboxylic acid to yield the final product as a yellowish solid (16 mg, 37% yield). MS (apci) m/z=451.2 (M+H).

Example 57

(R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-1-methyl-6-oxo-1,6-dihydropyridazine-3-carboxamide

Prepared according to Example 4, replacing 4-(methylsulfonamido)benzoic acid with 1-methyl-6-oxo-1,6-dihydropyridazine-3-carboxylic acid. The resulting light yellowish heavy suspension was vacuum-filtered, and the solid was rinsed with acetonitrile and ether, giving the first batch of pure product as a yellow powder (52 mg). A second batch of product was obtained through treating the concentrated filtrate from above with reverse-phase chromatography, eluting with 5 to 60% acetonitrile/water (total product from combining two batches: 65 mg, 91% yield). MS (apci) m/z=452.3 (M+H).

Example 58

(R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-4-methylpiperazine-1-carboxamide

Prepared according to the method of Example 2, replacing morpholine with 1-methylpiperazine, to yield the final product as a pale-yellowish foamy solid (4.5 mg, 63% yield). MS (apci) m/z=442.1 (M+H).

Example 59

(R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-4-hydroxy-4-(trifluoromethyl)piperidine-1-carboxamide

Prepared according to Example 13, replacing azetidin-3-ol hydrochloride with 4-(trifluoromethyl)piperidin-4-ol, to yield the final product as a pale-yellowish solid (35 mg, 86% yield). MS (apci) m/z=511.2 (M+H).

Example 60

(R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1, 2-b]pyridazin-3-yl)-2,2,2-trifluoroacetamide

A DCM (1 mL) solution of (R)-6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-amine (Preparation B; 50 mg, 0.16 mmol) was cooled in an ice bath, followed by addition of 2,2,2-trifluoroacetic anhydride (24 μl, 0.17 mmol) and pyridine (14 μl, 0.17 mmol) drop-wise. The ice bath was removed after reagent addition and the reaction was warmed to ambient temperature. After stirring for one hour, the reaction was concentrated and directly purified by reverse-phase column chromatography, eluting with 5 to 70% acetonitrile/water to yield the final product as an off-white powder (45 mg, 69% yield). MS (apci) m/z=412.3 (M+H). 

What is claimed is:
 1. A compound selected from the group consisting of: (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)acetamide; (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-4-(methylsulfonamido)benzamide; (S)—N-(6-((R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-hydroxypyrrolidine-1-carboxamide; (R)—N-(6-((R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-hydroxypyrrolidine-1-carboxamide; (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-4-hydroxypiperidine-1-carboxamide; (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)piperidine-1-carboxamide; (R)-1-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-(4-methoxyphenyl)urea; (3R,4R)—N-(6-((R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3,4-dihydroxypyrrolidine-1-carboxamide; (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3,3-difluoropyrrolidine-1-carboxamide; (R)-tert-butyl 4-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-ylcarbamoyl)-2,2-dimethylpiperazine-1-carboxylate; (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-(hydroxymethyl)azetidine-1-carboxamide; and (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-1-methyl-6-oxo-1,6-dihydropyridazine-3-carboxamide; or a pharmaceutically acceptable salt thereof.
 2. The compound of claim 1, wherein the compound is (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)acetamide, or a pharmaceutically acceptable salt thereof.
 3. The compound of claim 1, wherein the compound is (R)—N-(6-(2-(2, 5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-4-(methylsulfonamido)benzamide, or a pharmaceutically acceptable salt thereof.
 4. The compound of claim 1, wherein the compound is (S)—N-(6-((R)-2-(2, 5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-hydroxypyrrolidine-1-carboxamide, or a pharmaceutically acceptable salt thereof.
 5. The compound of claim 1, wherein the compound is (R)—N-(6-((R)-2-(2, 5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-hydroxypyrrolidine-1-carboxamide, or a pharmaceutically acceptable salt thereof.
 6. The compound of claim 1, wherein the compound is (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-4-hydroxypiperidine-1-carboxamide, or a pharmaceutically acceptable salt thereof.
 7. The compound of claim 1, wherein the compound is (R)—N-(6-(2-(2, 5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)piperidine-1-carboxamide, or a pharmaceutically acceptable salt thereof.
 8. The compound of claim 1, wherein the compound is (R)-1-(6-(2-(2, 5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-(4-methoxyphenyl)urea, or a pharmaceutically acceptable salt thereof.
 9. The compound of claim 1, wherein the compound is (3R,4R)—N-(6-((R)-2-(2, 5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3,4-dihydroxypyrrolidine-1-carboxamide, or a pharmaceutically acceptable salt thereof.
 10. The compound of claim 1, wherein the compound is (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3,3-difluoropyrrolidine-1-carboxamide, or a pharmaceutically acceptable salt thereof.
 11. The compound of claim 1, wherein the compound is (R)-tert-butyl 4-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-ylcarbamoyl)-2,2-dimethylpiperazine-1-carboxylate, or a pharmaceutically acceptable salt thereof.
 12. The compound of claim 1, wherein the compound is (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-(hydroxymethyl)azetidine-1-carboxamide, or a pharmaceutically acceptable salt thereof.
 13. The compound of claim 1, wherein the compound is (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-1-methyl-6-oxo-1,6-dihydropyridazine-3-carboxamide, or a pharmaceutically acceptable salt thereof.
 14. The compound of claim 1, wherein the compound is selected from the group consisting of: (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)acetamide; (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-4-(methylsulfonamido)benzamide; (S)—N-(6-((R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-hydroxypyrrolidine-1-carboxamide; (R)—N-(6-((R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-hydroxypyrrolidine-1-carboxamide; (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-4-hydroxypiperidine-1-carboxamide; (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)piperidine-1-carboxamide; (R)-1-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-(4-methoxyphenyl)urea; (3R,4R)—N-(6-((R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3,4-dihydroxypyrrolidine-1-carboxamide; (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3,3-difluoropyrrolidine-1-carboxamide; (R)-tert-butyl 4-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-ylcarbamoyl)-2,2-dimethylpiperazine-1-carboxylate; (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-(hydroxymethyl)azetidine-1-carboxamide; and (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-1-methyl-6-oxo-1,6-dihydropyridazine-3-carboxamide.
 15. The compound of claim 1, wherein the compound is in the form of a pharmaceutically acceptable salt.
 16. A pharmaceutical composition comprising a compound selected from the group consisting of: (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)acetamide; (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-4-(methylsulfonamido)benzamide; (S)—N-(6-((R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-hydroxypyrrolidine-1-carboxamide; (R)—N-(6-((R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-hydroxypyrrolidine-1-carboxamide; (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-4-hydroxypiperidine-1-carboxamide; (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)piperidine-1-carboxamide; (R)-1-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-(4-methoxyphenyl)urea; (3R,4R)—N-(6-((R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3,4-dihydroxypyrrolidine-1-carboxamide; (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3,3-difluoropyrrolidine-1-carboxamide; (R)-tert-butyl 4-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-ylcarbamoyl)-2,2-dimethylpiperazine-1-carboxylate; (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-(hydroxymethyl)azetidine-1-carboxamide; and (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-1-methyl-6-oxo-1,6-dihydropyridazine-3-carboxamide; or a pharmaceutically acceptable salt thereof; and a pharmaceutically acceptable diluent or carrier.
 17. The pharmaceutical composition of claim 16, wherein the compound is (R)—N-(6-(2-(2, 5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)acetamide, or a pharmaceutically acceptable salt thereof.
 18. The pharmaceutical composition of claim 16, wherein the compound is (R)—N-(6-(2-(2, 5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-4-(methylsulfonamido)benzamide, or a pharmaceutically acceptable salt thereof.
 19. The pharmaceutical composition of claim 16, wherein the compound is (S)—N-(6-((R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-hydroxypyrrolidine-1-carboxamide, or a pharmaceutically acceptable salt thereof.
 20. The pharmaceutical composition of claim 16, wherein the compound is (R)—N-(6-((R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-hydroxypyrrolidine-1-carboxamide, or a pharmaceutically acceptable salt thereof.
 21. The pharmaceutical composition of claim 16, wherein the compound is (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-4-hydroxypiperidine-1-carboxamide, or a pharmaceutically acceptable salt thereof.
 22. The pharmaceutical composition of claim 16, wherein the compound is (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)piperidine-1-carboxamide, or a pharmaceutically acceptable salt thereof.
 23. The pharmaceutical composition of claim 16, wherein the compound is (R)-1-(6-(2-(2, 5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-(4-methoxyphenyl)urea, or a pharmaceutically acceptable salt thereof.
 24. The pharmaceutical composition of claim 16, wherein the compound is (3R,4R)—N-(6-((R)-2-(2, 5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3,4-dihydroxypyrrolidine-1-carboxamide, or a pharmaceutically acceptable salt thereof.
 25. The pharmaceutical composition of claim 16, wherein the compound is (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3,3-difluoropyrrolidine-1-carboxamide, or a pharmaceutically acceptable salt thereof.
 26. The pharmaceutical composition of claim 16, wherein the compound is (R)-tert-butyl 4-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-ylcarbamoyl)-2,2-dimethylpiperazine-1-carboxylate, or a pharmaceutically acceptable salt thereof.
 27. The pharmaceutical composition of claim 16, wherein the compound is (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-(hydroxymethyl)azetidine-1-carboxamide, or a pharmaceutically acceptable salt thereof.
 28. The pharmaceutical composition of claim 16, wherein the compound is (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-1-methyl-6-oxo-1,6-dihydropyridazine-3-carboxamide, or a pharmaceutically acceptable salt thereof.
 29. The pharmaceutical composition of claim 16, wherein the compound is selected from the group consisting of: (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)acetamide; (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-4-(methylsulfonamido)benzamide; (S)—N-(6-((R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-hydroxypyrrolidine-1-carboxamide; (R)—N-(6-((R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-hydroxypyrrolidine-1-carboxamide; (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-4-hydroxypiperidine-1-carboxamide; (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)piperidine-1-carboxamide; (R)-1-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-(4-methoxyphenyl)urea; (3R,4R)—N-(6-((R)-2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3,4-dihydroxypyrrolidine-1-carboxamide; (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3,3-difluoropyrrolidine-1-carboxamide; (R)-tert-butyl 4-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-ylcarbamoyl)-2,2-dimethylpiperazine-1-carboxylate; (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-3-(hydroxymethyl)azetidine-1-carboxamide; and (R)—N-(6-(2-(2,5-difluorophenyl)pyrrolidin-1-yl)imidazo[1,2-b]pyridazin-3-yl)-1-methyl-6-oxo-1,6-dihydropyridazine-3-carboxamide.
 30. The pharmaceutical composition of claim 16, wherein the compound is in the form of a pharmaceutically acceptable salt. 